1. Field of the Invention
The invention concerns a jamming device for rope and alike whose casing consists of two parallelly, one above the other, situated flanges. Between the said flanges are perpendicularly mounted two pivoting pulleys and a jamming cleat.
A similar jamming device, used chiefly in alpinism for ascending and descending along the rope, is known under FR-PS 2,451,752. On at least one of its flanges are mounted a first and a second pivoting pulleys, by means of which occurs during ascending and/or descending a relative motion between the rope and the said pulleys.
Fastened to the casing is also a fixed pulley along which is wound the rope. To effect a jamming position between the fixed pulley and the first pivoting pulley is the fixed pulley in functional relationship with the first pivoting pulley. The latter, however, is coupled with the operating handle which is pivotally mounted between the two flanges. The first pivoting pulley is additionally equipped with two bosses facing the fixed pulley. These bosses wedge, in association with the fixed pulley, the rope in two jamming positions.
To make threading of the rope into the jamming device easier is one of the flanges retractable, thus allowing the rope to be wound around the pivoting pulleys and in particular between the first pivoting pulley and the fixed pulley.
If the user wishes to descend he embraces the handle with hand and pushes it in the middle position. A relative motion between the rope and the individual components of the jamming device ensues when neither of the first pivoting pulley's bosses blocks the rope between the said bosses and the fixed pulley--that is to say, when the rope is allowed to slide smoothly between the first fixed pulley and the first pivoting pulley. The first jamming position is activated by pressing the handle against the casing. Thus the user causes the first boss of the first pivoting pulley to edge the rope between the first pivoting pulley and the fixed pulley.
The second jamming position is activated by pushing the handle away from the casing until the angle of about 90.degree. has been reached; then the rope is wedged between the second boss of the first pivoting pulley and the fixed pulley.
If the user releases the operating handle he causes the first pivoting pulley to rotate anti-clockwise until the operating handle gets pressed closely against the casing. The weight of user's body namely effects in the first pivoting pulley a moment of anti-lockwise rotation, activating so automatically the first jamming position of the rope.
A drawback of the jamming device under consideration is the occurrence that the first jamming position of the rope gets activated only when the operating handle is pressed firmly against the casing. Therefore the user is forced to remove the fingers he has between the handle and the casing, and then press with hand the handle against the casing if he wishes to wedge the rope safely in the first jamming position. If it happens--say, in the case of accident--that the user is unable to press the handle against the casing, the moment of rotation generated in the first pivoting pulley will activate the first jamming position in which the rope is wedged between the first boss and the fixed pulley. However, the weak point of this first jamming position is that it does not wedge the rope safely, for the jamming force generated by the first boss and the fixed pulley depends on the moment of rotation created in the first pivoting pulley as well as on the magnitude of force of friction between the rope, the fixed pulley and the boss. If the jamming force is smaller than the force of friction the jamming device does not hold the user firmly in place, and thus he slides downwards. This happens particularly if the rope's diameter is too small and/or the rope's surface is too smooth.
As to the second jamming position, its disadvantage is that the required angle between the handle and the casing is about 90.degree.. This implies that great physical strength is required from the user not only to activate the second jamming position but also to retain it, for he has to overpower all the time the moment of rotation generated by the weight of user's body in the first pivoting pulley.
The said large opening angle of 90.degree. has for the user another adverse consequence: the contact of user's hand with the casing gets lost immediately after the opening angle of the handle has reached the distance exceeding the span of his hand.